The bold line should have disappeared after instance shutdown, but it didn’t. From “natcch” (number of attached processes) column I see there is still some process using the shared memory segment. Thus the segment was not released and even ipcrm command did not remove it (just like with normal files if someone has them open).

So, I needed to identify which process was still using the memory segment. If that had been a normal existing file, I’d could have used /sbin/fuser command to see which process still holds it open, but this only works for existing files with existing directory entries.

However for deleted files, sockets and shared memory segments, you can use lsof command (it’s normally installed by default on Linux, but for Unixes you need to separately download and install).

The SHM ID of that segment was 393216 as ipcs -ma showed, so I simply run lsof to show all open file descriptors and grep for that SHM ID:

Note that the lsof command is very useful for many other tasks as well. For example it allows you to list open sockets by network protocol, IP, port etc. For example you can determine to which client some server process is talking to, from OS level:

Unfortunately lsof is not installed by default in classic Unixes, but in some shops the sysadmins have chosen to install it. But even then, it may not work for regular users as lsof requires access to kernel memory structures through /dev/kmem or similar. If you can’t get access to lsof then there may be other tools available which can do some tricks lsof can do. For example on Solaris, there’s an useful command pfiles which can show open files of a process and since Solaris 9 ( I think ) it can also report the TCP connection endpoints of network sockets…